1. Field of the Invention
The present invention relates to a semiconductor device having a silicon nitride film and, more particularly, to a semiconductor device having a silicon oxide film.
2. Description of the Background Art
As a process of depositing a silicon nitride film on a semiconductor substrate such as a silicon substrate, a process of nitriding the silicon substrate from its surface and a process of depositing a silicon nitride film by chemical vapor deposition (hereinafter referred to as CVD) are dominantly used. Of these processes, catalytic chemical vapor deposition (hereinafter referred to as catalytic CVD) is useful. In catalytic CVD, source gases including monosilane gas and ammonia gas are supplied through a heated catalyzer onto a silicon substrate to deposit a silicon nitride film onto the silicon substrate.
Processes for depositing a silicon nitride film using this catalytic CVD will be described using FIG. 11. An apparatus for depositing a nitride film by using this catalytic CVD will be described first This film depositing apparatus includes a reaction chamber 60 in which a nitride film is deposited on a semiconductor substrate 51. In the reaction chamber 60, a substrate holder 52 holds the semiconductor substrate 51, a gas supply unit 54 supplies source gases 55 including silane gas and ammonia gas onto the semiconductor substrate 51, and a catalyzer 56 is interposed between the gas supply unit 54 and the substrate holder 52. It is noted that the catalyzer includes a metal heated by an electric current flowing therethrough. A carbon sheet 53 is sandwiched between the substrate holder 52 and the semiconductor substrate 51 to improve thermal contact. The film depositing apparatus has also gas tanks storing respectively the source gases including the silane gas and the ammonia gas and a gas tube connecting the gas tanks and a gas supply unit in the reaction chamber. The source gases are guided from the gas tanks through the gas tube and the gas supply unit into the reaction chamber.
Next, catalytic CVD for deposition of a nitride film will be described below. The process is performed in the following steps.
(a) A semiconductor substrate 51 is placed on the carbon sheet 53 arranged on the substrate holder 52 in the reaction chamber 60.
(b) The source gases 55 including silane gas and ammonia gas are guided from the gas tanks (not shown) storing respectively the source gases into the reaction chamber 60 through a gas tube 65 and the gas supply unit 54.
(c) The source gases 55 guided into the reaction chamber 60 are brought into contact with the heated catalyzer 56 and then supplied onto the semiconductor substrate 51.
(d) A silicon nitride film is deposited on the semiconductor substrate 51 from the supplied source gases 55.
The above process depositing of a silicon nitride film by using the conventional catalytic CVD has the following problems.
(1) The ammonia gas of the source gases had to be supplied at a flow rate about 100 times the flow rate of silane gas. Since the ammonia gas is bad for the environment, the flow rate of ammonia gas must be reduced.
(2) The thickness uniformity of an obtained silicon nitride film in the plane of the substrate 51 is poor. For example, when a silicon substrate has a diameter of 10.16 cm (4 inches), the thickness uniformity in the plane is poor, i.e., about 13%.
(3) Since the thermal contact between the semiconductor substrate 51 and the substrate holder 52 is insufficient, the semiconductor substrate 51 is overheated by heat radiation from the heated catalyzer 56. In particular, a compound semiconductor device using a GaAs substrate or the like cannot be placed at an excessively high temperature. More specifically, substrate temperature must be about 360° C. or lower in order to prevent an ohmic electrode from being deteriorated. On the other hand, when a gap between the catalyzer and the semiconductor substrate is set to prevent the semiconductor substrate from being overheated, the film deposition rate decreases. The substrate temperature must be 300° C. or higher in the deposition of the silicon nitride film to remove absorbed water in the substrate surface.